UNIVERSITY   OF   CALIFORNIA 

COLLEGE    OF    AGRICULTURE 

AGRICULTURAL    EXPERIMENT    STATION 

BERKELEY,    CALIFORNIA 


CITRUS  CULTURE  IN  CENTRAL 
CALIFORNIA 

GORDON  J.  SURR  and  L.  D.  BATCHELOR 


BULLETIN  405 

August,  1926 


UNIVERSITY  OF  CALIFORNIA  PRINTING  OFFICE 

BERKELEY,  CALIFORNIA 

1926 


Digitized  by  the  Internet  Archive 

in  2012  with  funding  from 

University  of  California,  Davis  Libraries 


http://www.archive.org/details/citruscultureinc405surr 


CITRUS   CULTURE   IN    CENTRAL  CALIFORNIA* 

GORDON  J.  SURRt  and  L.  D.  BATCHELORJ 


INTRODUCTION 

Upon  request  of  the  citrus  growers  of  Tulare  County,  California, 
cooperative  field  trials  were  conducted  by  the  Citrus  Experiment 
Station  in  Tulare  County  during  the  five  years  from  1919  to  1924. 
The  citrus  growers  generally,  through  various  fruit-shipping  associa- 
tions, and  the  individual  grove  owners  on  whose  property  the  trials 
were  located,  financially  supported  the  investigation,  and  the  senior 
author  was  continually  in  charge  of  the  field  work.1 

The  following  pages  summarize  the  results  of  these  experiments 
and  of  numerous  observations  in  other  citrus  groves.  Based  upon 
these  results  a  system  of  soil  management  is  suggested  which  may 
possibly  be  applied  beyond  the  confines  of  the  area  where  the  data 
were  collected.  Many  of  the  recommendations  and  conclusions  which 
follow  are  in  harmony  with  and  have  been  set  forth  as  a  result  of 
other  similar  investigations  elsewhere.  In  writing  this  account  the 
published  papers  by  Vaile  and  Surr,  reporting  on  these  trials,  have 
been  utilized. 

It  was  realized  at  the  outset  that  the  problems  to  be  studied,  such 
as  low  production  and  lack  of  vitality  of  citrus  trees,  were  more  in 
evidence  on  adobe  than  on  other  soils,  hence  the  field  trials  were 
located  on  this  soil  type.  Many  observations  were  also  made  in  citrus 
groves  on  other  soils  and  numerous  soil  moisture  determinations  were 
made  in  connection  with  irrigation  studies.  Commencing  with  the 
spring  of  1923,  nitrate  determinations  were  made  of  soil  samples 
taken  from  the  experimental  plots,  and  this  work  was  continued 
until  the  fall  of  1924. 

HISTORY  OF   GROVES    USED    FOR   THE    EXPERIMENTS 

One  of  the  experimental  groves  was  located  on  the  Ball-Emery 
property,  about  five  miles  southeast  of  Porterville.  The  trees,  which 
are  Washington  Navels  on  sour  stock,  were  planted  in   1912,   and 

*  Paper   No.    150,    University    of   California,    Graduate    School    of    Tropical 
Agriculture  and  Citrus  Experiment  Station,  Eiverside,  California, 
t  Field  Assistant  in  Citrus  Experiment  Station. 

X  Professor  of  Orchard  Management,  Horticulturist  in  Agricultural  Experi- 
ment Station. 

1  The  authors  wish  to  acknowledge  the  painstaking  care  with  which  Prof. 
R.  S.  Vaile  planned  the  series  of  experiments  herein  reported,  and  the  service 
he  rendered  in  supervising  them  during  the  first  three  years  of  the  cooperative 
arrangement. 


4  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

although  apparently  healthy,  they  were  small  for  their  age,  and  had 
not  usually  borne  satisfactory  crops.  It  is  believed  that,  in  the  past, 
inadequate  water  supply  was  one  of  the  factors  which  had  limited 
the  yields  in  this  grove.  This  area  is  now  in  an  irrigation  district 
and  water  will  probably  be  more  abundant  in  the  future.  In  the 
block  selected  for  the  field  trials  seven  plots  of  from  seventy-two  to 
seventy-five  trees  each  were  laid  out. 

A  second  experimental  grove  was  located  on  the  property  of  the 
Kaweah  Lemon  Company  at  Lemon  Cove.  Here,  the  trees,  which 
are  Washington  Navels  on  sweet  stock,  were  planted  in  1904.  The 
irrigation  water  supply  from  the  Kaweah  River  was  generally  abun- 
dant. Seven  plots,  of  from  forty-one  to  fifty-five  trees  each,  were  laid 
out  in  this  grove.  The  trees  are  larger  and  older  than  those  on  the 
Porterville  plots  and  many  were  in  an  unproductive  condition  when 
the  experiments  were  started. 

The  soil  of  the  former  grove  is  classified  as  Porterville  Adobe, 
while  that  of  the  latter  is  classified  as  Olympic  Adobe.  The  wilting 
point  of  the  soil  in  the  Lemon  Cove  grove  is  from  sixteen  to  nearly 
twenty-two  per  cent.  The  soil  shrinks,  upon  air-drying,  to  about 
half  of  its  volume  when  wet. 

The  following  cultural  treatments  were  included  in  both  trials: 

CULTURAL   TREATMENTS 

Plan  of  Treatments. — All  the  plots  were  fertilized  in  a  similar 
way,  with  minor  exceptions  noted  later.  The  seven  differential  cul- 
tural treatments  were  as  follows: 

1.  Cover-cropped  in  summer,  fallow  in  winter. 

2.  Straw  mulch  in  summer,  fallow  in  winter. 

3.  Cultivated  in  summer,  straw  plowed  under  in  fall,  fallow  in 

winter. 

4.  Plowed  several  times  during  summer,  with  little  or  no  other 

tillage,  fallow  in  winter. 

5.  Intercropped  with  alfalfa  in  every  other  inter-space.     Clean 

cultivation  in  alternate  inter-spaces. 

6.  Clean  cultivation  in  summer,  legume  cover-crop  in  winter. 

7.  Rotation  of  the  above  treatments  except  No.  5. 
Fertilization, — While   adobe   soils   are   commonly   relatively   rich, 

citrus  trees  on  such  soils  generally  respond  to  proper  fertilization. 
It  was  decided,  from  past  experience,  to  fertilize  all  the  plots  from 
the  beginning.  Essentially  similar  materials  were  used  on  both  tracts 
but,  as  the  trees  were  older,  greater  amounts  were  applied  on  the 
Lemon  Cove  than  on  the  Porterville  groye. 


BULL.  405]  CITRUS    CULTURE   IN    CENTRAL    CALIFORNIA  5 

Barnyard  manure  was  applied  each  fall  and  some  commercial 
nitrogenous  fertilizer  each  spring.  The  applications  of  manure  were 
at  the  rate  of  ten  tons  per  acre  each  year  on  the  Lemon  Cove  orchard, 
and  about  seven  tons  per  acre  on  the  Porterville  orchard.  The  first 
year  some  of  the  manure  was  applied  in  furrows ;  later  it  was  always 
applied  broadcast. 

At  Lemon  Cove  the  spring  applications  of  commercial  fertilizer, 
since  1920,  varied  from  slightly  over  one  pound  to  over  two  pounds 
of  actual  nitrogen  per  tree,  while  the  spring  applications  at  Porterville 
were  from  half  a  pound  to  a  little  over  one  pound  of  actual  nitrogen 
per  tree.  The  materials  used  were  nitrate  of  lime,  sulphate  of 
ammonia,  tankage  reinforced  with  nitrate  of  ammonia,  nitrate  of 
soda,  and  in  one  year  a  so-called  "complete  fertilizer." 

Irrigation. — The  irrigation  practice  on  each  orchard  has  been  as 
nearly  uniform  as  possible.  In  general,  from  two  and  a  half  to  four 
acre  inches  of  water  per  acre  have  been  applied  monthly  during  the 
summer  season,  the  amount  having  been  increased  somewhat  where 
cover-crops  were  growing.  The  water  was  allowed  to  run  from 
twenty-four  to  a  maximum  of  seventy-two  hours  during  each  irri- 
gation. 

Cultivation. — Except  in  the  cases  of  the  winter-cover-cropped  plots 
the  manure  was  plowed  under  in  the  late  fall  or  early  winter.  The 
ground  was  left  rough  after  plowing.  Where  winter  cover-crops  were 
grown,  the  manure  was  disked  in  before  sowing  the  cover-crop  seed, 
and  the  cover-crop  was  plowed  under  the  following  spring.  The 
plots  which  were  clean-cultivated  received,  as  a  rule,  one  cultivation 
after  each  irrigation.  The  cultivation  given  was  generally  shallow. 
It  was  soon  found  that,  in  these  particular  orchards,  the  cracking  of 
the  adobe  soils,  on  drying  out  somewhat,  allowed  an  excellent 
penetration  of  water.  Consequently  the  ground  was  allowed  to  crack 
moderately  between  irrigations,  and  before  each  cultivation.  Fre- 
quently the  furrows  for  irrigation  were  made  several  days  before 
irrigating  in  order  to  promote  the  cracking  of  the  bottoms  of  the 
furrows. 

Changes  in  Treatment. — It  is  obvious  that  if  radical  changes  are 
made  after  starting  an  experiment,  it  will  be  difficult  or  impossible 
to  interpret  the  results.  However,  the  plots  with  alfalfa  growing  in 
alternate  middles  soon  began  to  show  that  this  treatment  was  harmful 
to  the  trees.  As  the  plots  were  kept  well  irrigated  it  was  apparently 
not  a  case  of  competition  for  water  between  the  trees  and  the  alfalfa. 
The  leaves  of  the  trees  became  yellow  and  sparse,  particularly  on  the 
Lemon  Cove  plot,  and  by  June,  1921,  the  trees  on  this  plot  seemed 


b  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

to  be  in  a  critical  condition.  Consequently  the  alfalfa  was  plowed 
under  on  both  plots  in  July,  1921,  and,  on  the  Lemon  Cove  plot,  two 
pounds  per  tree  of  sulphate  of  ammonia  was  applied.  Commencing 
with  the  spring  of  1922  the  original  alfalfa  plot  at  Lemon  Cove  was 
given  heavier  applications  of  commercial  fertilizer  than  the  other 
plots. 


YIELDS  OF  THE  LEMON   COVE   PLOTS 

Table  1  gives  the  average  yield  per  tree  in  field  boxes  of  approxi- 
mately forty-two  pounds  each,  on  the  Lemon  Cove  plots  for  the  years 
1919  to  1924,  inclusive.  The  yields  of  every  tree  were  recorded  each 
year,  but  all  diseased  trees,  replants  or  trees  severely  cut  back  were 
excluded.  The  table  also  shows  the  number  of  trees  counted  on  each 
plot,  and  the  average  annual  yield  per  tree  for  the  five  years  of  the 
experiment. 

TABLE  1 
Average  Annual  Yields  per  Tree  in  Field  Boxes  on  Lemon  Cove  Tract 


Yield 

in  field  boxes  per  tree 

Average 

Num- 

annual 

ber  of 

Treatment 

During 

yield 

Before 
trial 

per  tree 

during 

1919 

1920 

1921 

1922 

1923 

1924 

trial 

43 

Cover-cropped     in    summer, 

fallow  in  winter 

1.67 

2.90 

3.80 

2.34 

5.69 

0.58 

3.06 

50 

Straw     mulch     in     summer, 

*  1.36 

1.64 

2.40 

1.65 

7.01 

1.37 

2.81 

45 

Cultivated  in  summer,  straw 

plowed  under  in  fall,  fallow 

1.28 

1.54 

2.52 

1.26 

5.77 

0.97 

2.41 

55 

Plowed  several  times  during 

summer,  little  or  no  other 

cultivation,  fallow  in  winter.. 

1.38 

1.51 

2.14 

1.73 

6.43 

1.39 

2.64 

49 

Intercropped  with  alfalfa  in 
every  other   middle,   clean 
cultivation      in      alternate 

1.34 

2.55 

3.13 

1.93 

6.78 

1.30 

3.14 

41 

Clean  cultivation  in  summer, 

legume  cover-crop  in  winter.. 

1.62 

2.14 

3.54 

3.70 

7.26 

1.40 

3.61 

45 

Rotation  plot  receiving  one  of 
the  above  treatments  each 

year,  except  growing  alfalfa  . 

1.32 

2  03 

2.47 

1.31 

7.30 

1.10 

2.84 

In  the  first  place  it  will  be  noted  that  the  crop  harvested  in  1919, 
which  was  picked  in  November,  before  the  field  trials  were  started, 
differed  little  on  the  various  plots.  The  minimum  average  yield  was 
1.28  boxes  per  tree  and  the  maximum  1.67  boxes,  the  greatest  differ- 
ence having  been  therefore  only  a  little  over  a  third  of  a  field  box 


BULL.  405]  CITRUS    CULTURE   IN    CENTRAL    CALIFORNIA  7 

per  tree.  Expressed  in  per  cent,  using  the  highest  yield  as  100,  the 
lowest-yielding  plot  was  only  twenty-three  per  cent  below  the 
maximum. 

Weather  is  apt  to  greatly  influence  any  crop  grown  in  the  open 
air,  and  the  Navel  orange  crop  of  1922  in  the  Lemon  Cove  region 
was  considerably  smaller  than  the  1921  crop.  The  1923  season  was 
highly  favorable  to  citrus  production  in  Tulare  County,  probably 
largely  owing  to  the  direct  and  indirect  influence  upon  "June  drop" 
of  the  relatively  cool  spring  and  early  summer.  Whatever  the  causes, 
the  fact  remains  that  most  citrus  trees  in  Tulare  County,  unless 
obviously  in  poor  condition,  produced  a  good  crop  that  year.  The 
1923  yields  were  good  on  all  the  Lemon  Cove  plots. 

There  was  an  abrupt  falling  off  in  the  1924  yields  on  all  the  plots, 
apparently  largely  due  to  a  heavy  infestation  of  gray  Citrus  scale 
(Coccus  citricola).  Until  ths  infestation  the  trees  had  been  kept 
commercially  clean  by  spraying,  but  they  were  not  sprayed  in  1923. 
Moreover,  the  winter  of  1923-24  was  cold  and  dry.  Irrigation  water 
was  not  available  for  a  long  time  in  the  winter  and  early  spring. 
Throughout  the  Lemon  Cove  district  as  a  whole  the  Navel  orange 
crop  of  1924  was  considerably  smaller  than  the  1923  crop. 


COMMENTS  ON  THE  YIELDS    THE  TREATMENTS  AND  THE 

CONDITION    OF   THE   TREES    OF   THE 

LEMON    COVE    TRACT 

It  is  plain  that  a  moderate  application  of  fertilizer  has  had  a 
marked  effect  in  increasing  the  yields  of  all  the  plots  regardless  of 
the  cultural  treatment. 

In  average  yield  for  the  five  years  there  is  no  striking  difference 
between  any  of  the  plots.  Considering  the  winter  cover-crop  plot, 
which  produced  the  most  fruit,  as  100,  the  lowest-yielding  plot  was 
only  33  per  cent  less.  Such  a  difference  may  be  significant  in  this 
case,  but  it  must  also  be  realized  that  such  differences  are  frequently 
well  within  the  realm  of  chance  variations  in  plot  trials.  The  con- 
dition of  the  trees  at  the  end  of  the  experiment  should  also  be 
considered  as  well  as  the  yields  as  an  indication  of  the  effect  of  the 
several  cultural  treatments. 

The  summer-cover-crop  plot  yielded  more  than  the  other  plots  the 
first  year  but  was  only  slightly  ahead  the  second  year.  In  the  fourth 
and  fifth  years  the  yields  fell  off  decidedly  in  comparison  with  those 
of  the  other  plots.  In  the  fifth  year,  1924,  the  yield  was  much  below 
that  of  any  other  plot. 


8  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  summer  cover-crop  was  buckwheat  and  Melilotus  alba  sown 
together  in  1920  and  1921,  M .  alba  in  1922,  and  purple  vetch  in  1923 
and  1924.  Where  summer  cover-crops  are  regularly  grown,  weeds 
are  apt  to  gain  a  foothold,  and,  for  the  three  seasons,  from  1922  to 
1924,  the  summer  cover-crops  have  been  choked  by  a  rank  growth 
of  "water-grass."  At  the  close  of  1924  most  of  the  trees  were  in  a 
very  bad  condition  on  this  plot.  Competition  for  water  between  the 
trees  and  the  summer  cover-crops  apparently  was  not  a  factor  but 
competition  for  available  nitrogen  probably  was,  for  at  times  the 
nitrate  nitrogen  was  very  low  on  this  plot. 

The  plot  with  straw  as  a  mulch  in  summer,  and  the  plot  ivith  straw 
plowed  under  in  the  fall  have,  in  general,  been  relatively  low  in  yield. 

Among  the  objections  to  the  use  of  a  mulch  of  grain  straw  are  the 
danger  from  fire  and  the  growth  of  objectionable  weeds,  such  as 
Bermuda  grass,  which  seems  particularly  apt  to  get  a  good  start  while 
the  ground  is  mulched.  Further,  after  an  irrigation  or  two,  there  is 
likely  to  be  a  rank  growth  of  volunteer  grain,  making  an  undesirable 
summer  cover-crop.  The  use  of  grain  straw  at  Lemon  Cove  has  not 
been  worth  while,  whether  directly  plowed  under  or  as  a  summer 
mulch. 

On  these  plots  the  trees  have  shown  fairly  good  condition,  and 
in  certain  other  citrus  orchards  in  Tulare  County  on  adobe  soils  there 
has  seemingly  been  no  injury  from  the  use  of  straw.  Attention  is 
called  to  this  point  as  cases  are  well  known  elsewhere  on  other  soil 
types,  of  definite  injury  to  citrus  trees,  caused  by  applications  of 
grain  straw.  A  straw  mulch  in  winter  may  be  highly  injurious  to 
citrus  trees  even  on  adobe  soil.  For  example,  straw  was  plowed  under 
on  the  Lemon  Cove  plot  late  in  October,  1920.  Some  of  the  same 
straw  was  spread  on  an  adjoining  block  not  in  the  experiment  with 
the  intention  of  plowing  it  under  there  also,  but  rain  came  on  and 
the  straw  remained  all  winter  as  a  mulch.  The  leaves  of  the  trees 
on  this  area  turned  yellow  and  three  years  later  the  ill  effects  were 
still  apparent. 

The  plot  which  was  worked  almost  exclusively  with  a  plow  showed 
an  improvement  in  both  tree  and  soil  condition  sooner  than  any  other 
plot. 

The  yields  were  considerably  below  those  of  the  winter-cover-crop 
plot  for  the  first  three  years.  In  the  third  year  of  the  experiment 
(1922)  this  plowed  plot  yielded  less  than  half  as  much  as  the  winter- 
cover-crop  plot.  In  the  last  two  years  of  the  experiment,  however 
the  yields  of  the  plowed  plot  closely  approached  those  of  the  winter- 
cover-crop  plot. 


BULL.  405]  CITRUS    CULTURE   IN    CENTRAL    CALIFORNIA  9 

The  soil  rapidly  responded  to  the  plowing,  becoming  a  friable  loam 
rather  than  an  intractable  adobe,  and  during  the  five  years  it  was 
always  in  better  physical  condition  than  the  soil  of  the  other  plots. 
As  early  as  the  fall  of  1920,  only  a  few  months  after  the  first  plowing, 
the  trees  on  this  plot  showed  a  definite  response,  with  healthy,  dark 
green  foliage  and  a  general  appearance  of  vigor.  For  nearly  two 
years  the  trees  on  this  plowed  plot  looked  decidedly  better  than  those 
of  any  other  plot.  By  December,  1921,  however,  the  winter-cover-crop 
plot  was  equally  as  dark  green  and  vigorous  and,  from  this  time  to 
the  close  of  1924,  the  trees  on  these  two  plots,  as  a  rule,  appeared  to 
be  in  better  condition  than  those  on  any  other  plot. 

The  plot  with  the  growing  alfalfa  in  every  other  inter-space  during 
portions  of  1920  and  1921  yielded  slightly  more  fruit  in  1920  than 
the  average  of  all  plots  even  though  the  trees  were  in  a  very  devitalized 
condition.  The  larger  amounts  of  commercial  fertilizer  used  on  this 
plot  since  the  alfalfa  was  turned  under  have  so  far  not  been  reflected 
in  the  yields. 

As  already  stated,  the  growing  of  alfalfa  proved  injurious  to  the 
trees,  and  this  injury  was  apparently  not  due  to  competition  for 
water.  The  trees  showed  a  decided  improvement  soon  after  the  alfalfa 
was  turned  under. 

The  plot  which  was  cultivated  in  summer  and  sown  to  a  legume 
cover-crop  in  winter  had  a  greater  average  annual  yield  during  the 
five-year  period  than  any  other.  The  excess  over  some  of  the  others  was 
not  great,  but  the  crops  were  more  regular  and  this  was  the  only  plot 
which  gave  an  increased  yield  in  1922  compared  with  the  previous  year. 

The  winter  cover-crop  in  1920  and  1921  was  Melilotus  indica  sown 
in  September,  and  in  1922  and  1923  purple  vetch  also  sown  in  Sep- 
tember. Purple  vetch  proved  very  satisfactory  as  a  winter  cover-crop. 
Weather  permitting,  the  cover-crops  were  turned  under  not  later  than 
the  end  of  February. 

From  the  standpoints  of  both  yield  and  tree  condition  the  winter- 
cover-crop  plot  has  been  the  most  satisfactory  of  all  the  Lemon  Cove 
plots. 

The  rotation  plot  produced  an  average  annual  yield  practically 
the  same  as  the  average  of  all  plots  during  the  five-year  period.  This 
plot  had  a  summer  cover-crop  of  Melilotus  alba  and  buckwheat  in 
1920  a  winter  cover-crop  of  Melilotus  indica  in  1921,  clean  cultivation 
in  1922,  a  summer  mulch  of  four  and  a  half  tons  per  acre  of  alfalfa 
hay  in  1923,  and  clean  cultivation  during  1924.  The  trees  became 
somewhat  pale  during  the  late  summer  of  1920,  while  the  summer 
cover-crop  was  growing,  and  at  the  end  of  April,  1921,  were  still 
yellowish. 


10 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


YIELDS    OF    THE    PORTERVILLE    PLOTS 

The  treatments  on  the  Porterville  plots  were  essentially  the  same 
as  at  Lemon  Cove.  The  lowest-yielding  plot  before  the  experiments 
began  produced  thirty-seven  per  cent  less  than  the  highest-yielding 
plot. 

TABLE    2 
Average  Annual,  Yields  per  Tree  in  Field  Boxes  on  Porterville,  Tract 


Yield 

in  field  boxes  per  tree 

Treatment 

Average 

Num- 
ber of 
trees 

Before 
triil 
1919 

During  period  of  trial  (1920-1924) 

annual 

yield 

per  tree 

1920 

1921 

1922 

1923 

1924 

during 
trial 

74 

Cover-cropped     in     summer, 

fallow  in  winter 

0  81 

0.83 

0  63 

0.75 

2.19 

1.23 

1.13 

75 

Straw     mulch     in     summer, 

fallow  in  winter 

0.58 

1.07 

0.74 

0.70 

2.23 

1.86 

1.32 

72 

Cultivated  in  summer,  straw 
plowed  under  in  fall,  fallow 

in  winter 

0.81 

0.96 

0.70 

0  61 

2.19 

1.20 

1.13 

75 

Plowed  several  times  during 
summer,  little  or  no  other 

cultivation,  fallow  in  winter.. 

0.59 

0  85 

0.78 

0.60 

1.91 

0.92 

1.01 

74 

Intercropped   with  alfalfa  in 
every   other   middle,   clean 
cultivation      in      alternate 

middles 

0  90 

0.85 

0.41 

0.50 

1  37 

0.94 

0.81 

73 

Clean  cultivation  in  summer, 

legume  cover-crop  in  winter.. 

0  53 

0.94 

0.84 

0.84 

2.23 

0  84 

1.15 

75 

Rotation  plot  receiving  one  of 
the  above  treatments  each 

year,  except  growing  alfalfa  . 

0  57 

1.09 

0.81 

0  96 

2  05 

1.27 

1.24 

COMMENTS  ON  THE  YIELDS    THE  TREATMENTS  AND  THE 

CONDITION    OF   THE   TREES    OF   THE 

PORTERVILLE    PLOTS 

The  effectiveness  of  the  fertilizer  treatments  was  less  marked  on 
the  Porterville  plots  than  heretofore  noted  on  the  Lemon  Cove  plots. 
It  is  believed  that  a  shortage  of  irrigation  water  during  the  entire 
period  of  the  experiment  was  such  a  limiting  factor  in  production 
that  the  cultural  treatment  and  the  increased  fertilizer  applications 
were  largely  counteracted  by  this  unfavorable  condition.  The  yields 
showed  a  small  upward  trend  during  most  of  the  period  of  experi- 
mentation. The  plot  with  alfalfa  growing  in  every  other  inter-space 
showed  a  notable  falling  off  in  yield  compared  with  the  other  plots. 
It  is  doubtful  if  there  is  any  other  significant  difference  in  the  yields 
of  the  remaining  plots. 


BULL.  405]  CITRUS    CULTURE   IN    CENTRAL    CALIFORNIA  11 

The  summer -cover-crop  plot  was  sown  to  Melilotus  alba  in  1920, 
Virginia  soy  beans  in  1921,  Whippoorwill  cowpeas  in  1922  and  purple 
vetch  in  1923  and  1924.  The  summer  cover-crops  were  not  notably 
rank  and  were  always  free  from  water-grass.  The  trees  for  the  most 
part  remained  in  good  condition. 

The  practicability  of  the  cultural  treatments  was  similar  to  those 
of  the  Lemon  Cove  plots  heretofore  discussed.  Fig.  1  shows  a  view 
of  the  straw  mulched  plot,  directly  after  mulching.  This  treatment 
proved  impractical. 


FERTILZER    PRACTICES    FOR    CITRUS   ORCARDS    IN    CENTRAL 

CALIFORNIA 

The  following  suggestions  are  based  not  only  upon  the  experiments 
just  outlined,  but  also  upon  numerous  observations  in  other  orchards 
in  central  California. 

So  far,  in  both  central  and  southern  California,  organic  matter  and 
nitrogen  have  proved  to  be  of  paramount  importance  in  fertilizing 
citrus  trees.  An  excellent  fertilizer  program,  judging  from  results, 
is  to  apply  manure  in  the  fall  and  commercial  nitrogenous  fertilizer 
in  the  spring,  a  practice  which  has  been  generally  advised  by  Kelley2, 
Vaile  and  Surr3,  Hodgson4  and  others. 

When  manure  is  mixed  with  soil  the  nitrate  or  available  nitrogen 
temporarily  bceomes  unavailable  in  the  cells  of  organisms  decomposing 
the  manure.  From  this  standpoint,  and  also  for  convenience,  if 
winter  cover-crops  are  sown  in  the  autumn  the  manure  is  preferably 
applied  before  sowing  the  cover-crop. 

Citrus  trees  which  are  healthy  and  vigorous  tend  not  only  to  set 
good  crops  but  to  hold  a  good  proportion  of  the  fruit  in  the  June-drop 
period.  Every  effort  should  therefore  be  made  to  keep  them  in  the 
best  condition  possible  during  the  spring  and  early  summer.  Appli- 
cations of  manure  in  the  spring  would  tend  to  reduce  the  available 
nitrogen  temporarily  and  thus  lower  the  vitality  of  the  trees  at  what 
seems  to  be  a  critical  time. 

In  mature  citrus  orchards  ten  tons  of  barnyard  manure  to  the 
acre  annually  may  be  considered,  as  a  rule,  a  profitable  application. 


2  Kelley,  W.  P.  Our  present  knowledge  concerning  fertilization  of  citrus. 
Calif.  Citrograph,  76:  — .     1922. 

s  Vaile,  E.  S.,  and  Gordon  Surr.  Progress  report  of  orchard  management 
studies  in  Tulare  County.     Calif.  Citrograph,  9i:  — .     1923. 

4  Hodgson,  E.  W.  Fertilizing  citrus  trees  in  California.  California  Agr. 
Exp.  Sta.  Cir.  283:    1-22.     1925. 


12  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  use  of  manure  in  reasonable  quantities  each  year  is  recommended 
rather  than  occasional  applications  of  much  larger  amounts. 

Adobe  soils  are  commonly  slow  in  drying  out  in  the  spring  in 
central  California  and  after  a  rain  it  may  be  a  long  time  before  they 
can  be  cultivated  properly.  If  rain  immediately  follows  a  fertilizer 
application,  the  cover-crops  or  weeds  may  reap  the  benefit  of  the 
fertilizer  rather  than  the  trees.  This  has  happened  in  the  past,  the 
rank  growth  of  weeds  plainly  showing  where  the  fertilizer  had  been 
applied.  For  this  and  for  other  reasons  it  is  a  good  plan  to  disk 
a  winter  cover-crop  before  applying  the  fertilizer  and  to  plow  after 
the  application. 

As  nitrification  is  slow  in  cold  wet  soils,  the  nitrogen  in  the  spring 
applications  of  commercial  fertilizer  to  citrus  trees  on  adobe  soils 
should  preferably  be  in  part  or  wholly  in  the  nitrate  or  immediately 
available  form.  The  nitrates  most  commonly  used  include  nitrate 
of  lime,  nitrate  of  soda  and  nitrate  of  ammonia,  all  of  which  have 
given  excellent  results  on  adobe  soils,  when  used  in  conjunction  with 
barnyard  manure  and  cover-crops.  Light  soils,  however,  warm  up 
readily  and  fertilizers  may  be  used  on  them  in  which  the  nitrogen 
is  not  immediately  available,  such  as  dried  blood  or  sulphate  of 
ammonia. 

Where  a  mature  grove  receives  ten  tons  of  good  manure  to  the 
acre  annually,  about  one  pound  of  commercial  actual  nitrogen  to  the 
tree  should  suffice  in  most  cases  for  the  spring  application.  The 
profitable  amounts  of  commercial  fertilizer  which  may  be  applied 
depend,  however,  upon  various  factors,  such  as  the  quantity  and 
quality  of  the  manure  applied,  the  size  and  condition  of  the  trees, 
whether  good  winter  cover-crops  are  grown,  and  the  prices  prevailing 
for  the  fruit. 

There  is  a  popular  impression  that  manure,  used  alone,  tends  to 
make  citrus  fruits  coarse.  This,  however,  does  not  seem  to  be  the 
case  when  manure  is  used  in  reasonable  amounts. 

Manure  is  a  complete  fertilizer,  and  it  contains  the  principal 
elements  of  plant  food  in  proportions  much  nearer  to  those  actually 
taken  up  by  the  trees  and  plants  than  do  some  complete  high-phos- 
phorus, so-called  ' '  balanced, ' '  commercial  fertilizers.  Nitrogen,  unlike 
phosphorus  and  potash,  tends  to  be  leached  away  from  the  root  zone, 
which  is  one  reason  why  supplementing  manure  with  commercial 
nitrogen  is  generally  useful  in  citrus  groves. 

The  adobe  soils  planted  to  citrus  in  central  California  have  largely 
been  derived  from  gabbro  and  serpentine  rocks  that  are  relatively 
low  in  potash.     However,  potash  is  pretty  firmly  held  by  the  clayey 


BULL.  405]  CITRUS    CULTURE   IN    CENTRAL    CALIFORNIA  13 

materials  in  soils,  and  both  potash  and  phosphorus  are  of  course  added 
when  manure  is  applied.  Suffice  it  here  to  say  that,  from  what  has 
been  learned  to  date,  little  or  no  benefit  to  citrus  trees  in  central 
California  can  as  yet  be  definitely  ascribed  to  either  potash  or 
phosphorus  applied  in  commercial  fertilizers. 

The  Use  of  Lime,  Gypsum  and  Sulfur. — Vaile  and  Surr  endeav- 
ored to  show  in  1921  that  the  use  of  lime  was  in  general  not  worth 
while  in  citrus  groves  on  adobe  soils  in  Tulare  County5,  and  the  same 
seems  to  be  the  case  with  citrus  groves  on  other  soils  in  central 
California.  The  following  analyses  show  the  amount  of  lime  carbonate 
in  air-dried  soil  of  the  Lemon  Cove  plots :  1st  foot,  0.65  per  cent, 
which  is  equal  to  10  tons  per  acre  foot;  2nd  foot,  1.41  per  cent,  equal 
to  21  tons  per  acre  foot ;  3rd  foot,  2.38  per  cent,  equal  to  36  tons  per 
acre  foot;  and  4th  foot,  3.21  per  cent,  equal  to  48  tons  per  acre  foot. 
Soils  in  the  Porterville  region  are  reported  which  contain  from  12 
to  39  per  cent  lime  carbonate  in  the  3rd  foot6. 

The  three  applications  of  gypsum  on  the  Lemon  Cove  plots,  total- 
ing about  thirteen  and  a  half  tons  to  the  acre,  have  not  shown  any 
effect  on  tree  growth  or  yield  or  upon  the  growth  of  cover-crops  and 
weeds.  At  times  the  surface  soil  an  inch  or  two  in  depth  has  had 
a  better  texture  where  the  gypsum  was  applied.  On  the  other  hand, 
the  soil  on  the  plot  which  was  worked  almost  wholly  with  a  plow 
has  been  consistently  in  much  better  physical  condition  where  no 
gypsum  was  used  than  the  soil  on  the  other  plots  to  which  gypsum 
was  added.  The  same  is  true  of  the  applications  of  gypsum  to  the 
Porterville  plots. 

Sulfur  was  applied  to  orange  trees  in  Tulare  County  at  the  rate 
of  ten,  twenty,  and  forty  pounds  to  the  tree  in  1920,  but  no  material 
benefit  has  followed  the  application  of  this  material. 

Culls  and  Soil  Acidity. — The  question  is  frequently  asked  if  cull 
oranges  and  lemons  worked  into  the  soil  will  cause  soil  acidity.  There 
is  no  probability  at  all  of  any  acidity  arising  from  the  use  of  culls, 
of  which  over  twenty  tons  to  the  acre  were  applied  one  year  to  an 
orchard  in  Tulare  County.  The  acids  in  citrus  fruits  are  soon  broken 
down  by  soil  organisms,  and  it  was  found,  in  one  laboratory  experi- 
ment, that  citric  acid  itself  applied  at  the  rate  of  ten  tons  to  the  acre 
did  not  increase  the  acidity  of  the  soil.     Citrus  fruits  are  mostly 


s  Vaile,  E.  S.,  and  Gordon  Surr.  Use  of  various  compounds  of  calcium  on 
adobe  soils  of  the  foothill  regions  of  Tulare  County.  Calif.  Citrograph,  Vol. 
7i:   3,  24,  26,  27.     1921. 

6  Hilgard,  E.  W.  Marly  subsoils  and  chlorosis  or  yellowing  of  citrus  trees. 
California  Agr.  Exp.  Sta.  Circ.  27:    1-4.     1906. 


14  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

water  and  at  best  are  decidedly  low  in  fertilizing  value.  A  ton  of 
fresh  ripe  oranges,  for  example,  contains  on  an  average  only  3.66 
pounds  of  nitrogen,  1.10  pounds  of  phosphoric  acid  and  4.22  pounds 
of  potash. 


Fig.  1. — Straw  mulch,  five  tons  per  acre,  Ball-Emery  plot. 


IRRIGATION    PRACTICES 

It  would  be  folly  to  attempt  to  lay  down  hard  and  fast  rules  for 
the  irrigation  of  citrus  groves,  on  account  of  the  differing  conditions. 
On  adobe  soils  the  aims  should  be  to  let  the  ground  crack  moderately 
between  irrigations,  and  to  run  the  water  as  quickly  and  for  as  short 
a  time  as  possible  without  washing  the  soil.  When  soils,  particularly 
heavy  soils,  are  wet  for  long  periods,  not  only  do  the  roots  of  citrus 
trees  cease  to  function  but  in  many  cases  they  actually  die  and  rot. 
Figures  2  and  3  are  photographs  of  Navel  orange  trees  on  adobe  soil, 
just  below  and  just  above  an  irrigation  pipe-line  in  the  same  grove. 
Both  photographs  were  taken  from  the  same  spot.  The  ground  just 
below  the  pipe-line  has  been  wet  for  lengthy  periods  with  disastrous 
consequences  to  the  trees. 

In  dealing  with  citrus  orchards  on  open  soils  and  subsoils  it  should 
be  borne  in  mind  that,  when  large  amounts  of  irrigation  water  are 
applied,  water  and  soluble  plant  food  materials  may  be  carried  below 
the  root-zone,  in  which  event  they  are  practically  wasted. 

For  the  best  results  from  every  point  of  view,  including  economy 
in  the  use  of  water,  the  irrigation  pipe-lines,  in  a  citrus  grove  on  any 
type  of  soil  should  not  be  much  over  three  hundred  feet  apart. 


Bull.  405] 


CITRUS    CULTURE    IN    CENTRAL    CALIFORNIA 


15 


Fig.  2. — Navel  orange  trees  with  rotten  roots  on  adobe  soil  just 
below  the  irrigation  pipe-line. 


Fig.  3. — Healthy  Navel  orange  trees  in  the  same  grove  just  above  the 
pipe-line.     Both  photographs  taken  from  the  same  spot. 


16  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 

In  some  orchards  there  are  wide  strips  in  the  tree  rows  which 
get  no  water  during  the  whole  irrigation  season.  Yet  the  winter  rains 
wet  these  areas  and  generally  cause  the  roots  therein  to  become  active 
in  the  spring,  which  activity  persists  as  long  as  moisture  is  available. 
One  of  the  best  ways  at  present  known  of  reducing  the  ravages  of 
June-drop  is  to  keep  citrus  trees  in  the  best  possible  condition,  and 
it  is  obvious  that  if  many  roots  cease  to  function,  owing  to  the  drying 
out  of  the  soil  in  the  tree  rows,  the  vigor  of  the  trees  may  be 
correspondingly  impaired  at  what  seems  to  be  a  critical  time. 

It  would  appear  desirable  to  change  the  positions  of  the  irrigation 
furrows,  instead  of  having  them  in  exactly  the  same  places  at  each 
irrigation,  and  it  is  a  good  plan  to  make  the  furrows  a  few  days 
before  applying  the  water  on  adobe  soils,  thus  aiding  the  bottoms  of 
the  furrows  to  crack. 

Finally,  it  is  well  to  remember  that  over-irrigation  is  apt  to  cause 
far  more  serious  injury  to  citrus  trees  than  under-irrigation. 


CULTIVATION     PRACTICES 

Veihmeyer,  and  Beckett,  of  the  University  of  California,  found 
that,  under  field  conditions,  the  differences  in  the  losses  of  moisture, 
between  cultivated  and  uncultivated  soils  were  negligible,  provided 
that  weeds  were  not  allowed  to  grow.7  Much  harm  is  often  done  by 
cultivating  orchards  when  the  ground  is  too  wet.  When  it  is  fully 
realized  that  cultivation,  in  itself,  is  of  little  or  no  avail  in  conserving 
moisture,  it  is  hoped  that  there  will  be  less  haste  in  working  the  soil 
after  irrigation  or  rain. 

Cultivation  is  necessary  to  kill  weeds  and  to  turn  under  cover - 
crops  and  fertilizers,  and  certain  soils  do  not  take  water  well  without 
some  cultivation.  Cultivation,  furthermore,  is  apt  to  stimulate  nitrifi- 
cation, particularly  in  heavy  soils. 

Some  citrus  groves  are  plowed  once  a  year,  some  several  times  a 
year,  and  some  not  at  all,  with  equally  good  yields  and  tree  condition. 
Plowing,  however,  indirectly  much  improves  the  texture  of  adobe 
soils  and  it  will  be  of  interest  to  briefly  discuss,  the  reasons.  Adobe 
soils  swell  on  wetting  and  shrink  on  drying  and  when  a  slice  of  adobe 
soil,  turned  over  with  a  plow,  is  soaked  by  water,  expansion  takes 
place  to  a  marked  degree  as  there  is  little  or  no  side-pressure  to 
interfere  with  the  swelling.  On  drying  out,  the  greatly  expanded  soil 
falls  apart  and  crumbles  into  small  particles.    Repeated  plowings  may 


7  Eeport  of  the  College  of  Agriculture  and  the  Agricultural  Experiment  Station 
of  the  University  of  California,  July  1,  1921,  to  June  30,  1922,  p.  105. 


BULL.  405]  CITRUS    CULTURE   IN    CENTRAL    CALIFORNIA  17 

have  little  effect,  however,  on  the  texture  of  soils  which  do  not  swell 
much  on  wetting.  In  this  connection  it  is  of  interest  to  note  that 
both  the  plots  on  adobe  soil,  which  were  frequently  plowed,  in  the 
Tulare  County  trials,  had  a  higher  content  of  nitrate  nitrogen  than 
the  plots  receiving  ordinary  cultivation,  including  plowing  once  a 
year. 

Many  orchards  are  cultivated  more  than  is  either  necessary  or 
desirable  and  great  improvements  in  soil  conditions  have  occurred 
after  reducing  the  amount  of  cultivation,  in  fact  there  are  productive 
citrus  groves,  with  excellent  soil  conditions,  which  receive  very  little 
cultivation.  When  orchards  have  remained  uncultivated  all  winter, 
with  regular  cultivation  throughout  the  irrigation  season,  the  earlier 
irrigations  may  be  efficient  but,  in  many  cases,  the  penetration  of  the 
water  becomes  increasingly  poor  as  the  season  advances.  The  more 
frequent  the  cultivation  the  more  the  soil  may  be  packed  with  the 
formation  of  so-called  "plow-sole,"  which  condition,  however,  in  a 
number  of  instances,  appears  to  be  due  in  part  to  irrigation. 

The  system  of  cultivation  employed  is  largely  bound  up  with  the 
way  the  ground  takes  irrigation  water.  In  some  groves  irrigation  is 
satisfactory  after  running  the  water  several  times  in  the  same  ftirrows, 
and  in  other  orchards  it  may  be  advisable  to  make  new  furrows  before 
each  irrigation.  Such  problems  should  be  solved  by  each  grower 
himself,  by  means  of  trials,  made  preferably  on  a  small  scale  at  first. 

The  proper  depth  of  cultivation  likewise  depends  upon  circum- 
stances. The  roots  of  citrus  trees  have  occasionally  been  found  at 
considerable  depths,  but  the  feeding  root-zone  is  commonly  shallow  in 
heavy  soils  or  where  hardpan  is  near  the  surface.  Consequently, 
whatever  the  real  or  supposed  merits  of  deep  cultivation,  it  should 
not  be  practiced  in  citrus  groves  on  either  heavy  or  shallow  soils. 


COVER-CROP    PRACTICES 

Organic  matter  is  one  of  the  great  essentials  of  soil  fertility  and 
it  is  not  too  much  to  say  that  active  organic  matter  is  the  life  of  a 
soil,  the  term  active  here  signifying  organic  materials  which  decom- 
pose more  or  less  readily  under  favorable  conditions  when  incorporated 
with  the  soil. 

The  necessity  for  the  addition  of  organic  matter  has  long  been 
recognized  in  general  farming,  as  shown  by  the  common  inclusion 
of  a  green-manure  crop,  such  as  clover,  in  the  crop-rotation  program. 
The  growing  of  cover  or  green-manure  crops  in  orchards  has  also 


18  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

become  a  well  established  practice.  In  handling  such  crops,  however, 
certain  points  must  receive  attention  or  more  harm  than  good  may 
result. 

In  the  first  place  the  cover-crop  in  citrus  or  other  orchards  should 
be  a  legume  of  some  kind,  both  purple  vetch  and  Melilotus  indica 
being  largely  used  for  winter  cover-crops  in  citrus  groves  in  Cali- 
fornia. Legumes  can  obtain  nitrogen  from  the  air  but,  if  the  soil  is 
well  supplied  with  available  nitrogen,  the  growing  legumes  may  draw 
more  upon  the  soil  than  upon  the  air  for  their  nitrogen. 

However,  legumes  are  desirable  in  other  ways  as  cover-crops  on 
account  of  their  manner  of  decomposition  when  turned  under.  Green 
legumes  decompose  readily  under  suitable  conditions  and  soon  furnish 
available  nitrogen  to  the  trees,  unlike  wild  oats,  water-grass  and  cereals 
in  general  which  temporarily  render  nitrogen  unavailable  when  incor- 
porated with  soil.  It  is  believed  that,  in  many  cases,  citrus  yields  in 
central  California  have  been  considerably  reduced  by  rank  growths  of 
wild  oats  in  orchards.  Not  only  do  such  cover-crops  draw  heavily 
upon  the  soil  nitrogen  but  they  also  cause  the  nitrogen  to  be  unavail- 
able for  a  long  time  after  they  are  turned  under. 

Again,  legumes  may  have  a  decidedly  favorable  influence  upon  the 
texture  of  soils  on  account  of  the  "humus"  formed  during  their 
decomposition.  For  example,  Sievers  and  Holtz  state  that,  in  eastern 
Washington :  ' '  The  farmer  has  been  unable  to  influence  the  physical 
condition  of  his  soil  to  any  considerable  extent  through  the  application 
of  straw  or  strawy  manure  in  moderate  amounts  due  to  the  fact 
that  practically  no  humus  was  developed.  In  contrast  to  this  the 
growing  of  a  legume  crop  for  only  a  few  years  will  generally  benefit 
the  physical  condition  of  the  soil  so  decidedly  that  its  effects  are  felt 
for  several  years."8  The  roots  of  cover-crops  also  have  a  favorable 
mechanical  effect  upon  the  texture  of  soils  and  subsoils,  and  instances 
have  been  seen  in  central  California  and  elsewhere  of  marked  improve- 
ment in  the  physical  condition  of  soils  and  subsoils  following  the 
growing  and  turning  under  of  cover-crops. 

Cover-crops  also  largely  prevent  the  washing  away  of  the  surface 
soil  by  rains  and  the  leaching  below  the  root-zone  of  soluble  plant  food 
materials. 

Among  the  advantages  of  growing  cover-crops  in  winter  in  Cali- 
fornia, are  the  relative  dormancy  of  the  trees  and  the  utilization  of 
rain  instead  of  irrigation  water.  The  mistake,  however,  is  often  made 
of  sowing  the  winter  cover-crop  too  late  and  turning  it  under  too  late 


s  Sievers,  F.  J.,  and  H.  F.  Holtz.  The  silt  loam  soils  of  eastern  Washington 
and  their  management.     Washington  Agr.  Exp.  Sta.  Bull.  166:  40-42.     1922, 


Bull.  405] 


CITRUS    CULTURE   IN    CENTRAL    CALIFORNIA 


19 


in  the  spring.  Melilotus  indica  grows  slowly  in  cold  weather,  but  if 
sown  early,  say  about  September  1,  it  gets  a  good  start  while  the 
weather  is  warm  and  by  February  has  often  made  a  very  good  growth. 
Purple  vetch  grows  faster  in  cold  weather  than  Melilotus  indica  and 
has  made  a  very  satisfactory  winter  cover-crop  both  in  central  and 
in  southern  California. 


Fig.  4. — Summer  cover-crop  of  water-grass  in  Tulare  County  orange  grove. 

Winter  cover-crops  in  citrus  groves  in  central  California  should 
be  turned  under  not  later  than  the  end  of  February  or  early  in  March 
regardless  of  the  tonnage  of  the  cover-crops.  If  cover-crops  are 
turned  under  late  in  the  spring,  citrus  trees  may  lack  available  nitro- 
gen at  what  seems  to  be  a  critical  time,  with  a  consequent  reduction  in 
yield. 

One  of  the  benefits  of  a  winter  cover-crop  on  adobe  soils  may  be 
due  to  the  drying  out  of  the  soil  in  the  spring  by  the  growing  cover- 
crop,  thus  bringing  about  aeration,  and  encouraging  earlier  root 
activity  of  the  trees. 

In  young  citrus  groves  winter  cover-crops  may  furnish  all  the 
organic  matter  and  nitrogen  required,  but  such  is  not  usually  the 
case  in  the  older  orchards.  For  one  thing,  where  the  trees  are  large, 
it  is  often  difficult  to  grow  a  heavy  winter  cover-crop  on  account  of 
the  shade. 


20  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

In  some  citrus  groves  both  winter  and  summer  cover-crops  are 
grown  regularly,  but  many  growers  do  not  have  sufficient  water  for 
the  needs  of  both  summer  cover-crops  and  the  trees.  Instances  have 
been  seen  where  exceptionally  small  fruit  was  associated  with  the 
growing  of  summer  cover-crops.  This  may  have  been  in  part  an 
indirect  effect  of  the  poor  penetration  of  the  irrigation  water  in  the 
old  furrows.  Tests  were  made  in  the  Lemon  Cove  Laboratory  on 
samples  of  soil  from  orchards  in  central  California  in  which  summer 
cover-crops  were  growing  or  had  just  been  turned  under.  The  nitrate 
or  available  nitrogen  proved  to  be  extremely  low  under  rank  growths 
of  Melilotus  alba,  Sesbania,  and  water  grass,  and  quite  low  where 
cowpeas  or  purple  vetch  were  or  had  been  growing.  Weeds  are  apt 
to  get  a  good  start  in  cover-crops,  particularly  where  summer  cover- 
crops  are  grown  regularly.  The  accompanying  photograph  (figure  4) 
shows  a  heavy  growth  of  water  grass  (Echinochloa  crus-galli)  in  a 
Tulare  County  orchard  in  which  summer  cover-crops  of  cowpeas  have 
been  sown  for  several  seasons. 

In  young  citrus  orchards  both  summer  and  winter  cover-crops 
may  possibly  be  grown  with  advantage,  provided  the  water  supply 
is  sufficient.  In  this  way  the  soil  can  be  built  up  without  competition 
between  the  trees  and  the  growing  crops.  Later,  when  competition 
becomes  a  factor,  the  growing  of  summer  cover-crops  is  of  doubtful 
value. 


JUNE-DROP 

The  Navel  orange  is  especially  susceptible  to  June-drop,  which 
takes  a  heavy  toll  in  some  seasons.  Various  experiments  were  tried 
in  central  California  in  connection  with  June-drop.  For  example, 
in  1923,  a  number  of  trees  were  sprayed  with  lime-sulfur,  oil  and 
spreader,  and  with  Bordeaux  mixture.  Some  were  sprayed  when  in 
full  bloom,  others  ten  days  after  blooming  and  others  when  the  fruit 
was  from  one-half  to  three-quarters  of  an  inch  in  diameter,  and  there 
were  also  combinations  of  these  treatments.  Still  other  trees  were 
sprayed  with  whitewash  in  May,  1923.  However,  losses  from  June- 
drop  were  negligible  in  1923  and  the  yields  were  not  increased  by 
any  of  these  treatments. 

In  the  spring  of  1920  six  Navel  orange  trees  were  enclosed  in 
tents,  one  purpose  being  to  note  the  effects  upon  June-drop  of  the 
relatively  moister  air  inside  the  tents.  On  account  of  the  greater 
humidity   the   water   evaporated   through   a   porous   porcelain    bulb, 


BULL.  405]  CITRUS    CULTURE    IN    CENTRAL    CALIFORNIA  21 

inside  one  of  the  tents,  was  about  one-third  of  the  amount  evaporated 
through  a  similar  bulb  in  the  open  air.  As  thrips  like  dry  air  they 
caused  no  damage  whatever  to  the  fruit  in  any  of  these  tents,  although 
they  were  introduced  into  one  of  them.  In  spite  of  the  moister  air, 
June-drop  was  just  as  heavy  in  the  tents  as  outside  on  adjacent  trees. 

It  was  found  that  heavy  summer  cover-crops  reduced  the  evapora- 
tion of  water  from  porous  porcelain  bulbs  as  much  as  17  per  cent, 
compared  with  the  evaporation  from  similar  bulbs  on  clean-cultivated 
areas.  In  these  particular  trials,  however,  the  rank-growing  cover- 
crops  were  detrimental  to  the  orange  trees,  which,  in  their  weakened 
condition,  suffered  as  much  loss  from  June-drop  as  adjacent  trees  with 
clean  cultivation. 

The  effect  of  readily  available  nitrogenous  fertilizers  upon  June- 
drop  of  Navel  oranges  was  clearly  shown  by  the  fertilizer  trials  at 
Arlington.9 

Immediately  after  some  extremely  hot  weather  in  June,  1917, 
most  of  the  young  fruit  dropped  on  the  unfertilized  plots  as  well 
as  on  the  plots  receiving  manure  in  the  spring.  On  the  other  hand, 
the  plots  which  received  a  readily  available  nitrogenous  fertilizer 
in  the  spring  retained  a  medium  crop. 

One  of  the  best  ways  at  present  known  to  produce  good  crops  of 
Navels,  in  spite  of  June-drop,  is  to  keep  the  trees  in  the  best  possible 
condition  of  vigor  and  productivity,  particularly  from  early  spring 
until  after  the  June-drop  period.  With  adequate  fertilization  and 
proper  irrigation,  combined  with  efficient  pest  control,  June -drop  is 
not  apt  to  be  a  serious  problem  either  in  central  or  in  southern 
California, 

ROTTEN   ROOTS  AND   CITRUS   ROOTSTOCKS 

The  kind  of  stock  has  a  decided  bearing  on  the  occurrence  of 
root  rot.  When  heavy  soils  have  been  wet  for  long  periods,  citrus 
trees  on  such  soils  often  have  diseased  roots.  In  advanced  stages  of 
root  rot  the  foliage  becomes  scanty  and  of  light  color,  production 
ceases  and  the  trees  finally  die.  Whole  groves,  once  highly  productive, 
have  been  removed  for  this  reason.  Figure  5  is  a  photograph  of  an 
orange  tree  with  rotten  roots  in  adobe  soil.  In  some  cases,  marked 
improvement  has  followed  more  careful  irrigation,  stopping  leaks  in 
pipe  lines,  and  removing  the  soil  from  against  the  trunks  down  to 


9  Vaile,  E.  S.     Fertilizer  experiments  with  citrus  trees.     California  Exp.  Sta. 
Bull.  345:   1-47.     1922. 


22 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


the  crown  roots.  Where  these  things  have  been  done,  the  trees  in 
some  cases  have  grown  new  root  systems,  after  severe  cutting  back 
of  the  tops. 

On  account  of  the  removal  of  once  productive  citrus  orchards 
owing  to  root  rot  at  ages  when  they  should  be  in  their  prime,  the 
question  is  often  raised  as  to  the  future  of  the  citrus  groves  in 
central  California  on  adobe  soils.  Some  of  these  defunct  orchards 
were  heavily  irrigated,  with  water  in  the  furrows  for  lengthy  periods. 


.   : 


Fig.  5. — Orange  tree  with  rotten  roots  oi 


soil  in  Tulare  County. 


There  are  many  healthy  and  productive  citrus  orchards  on  adobe 
soils  in  this  region,  which  have  been  irrigated  with  only  moderate 
amounts  of  water,  with  no  indication  of  root  troubles. 

Some  of  the  older  citrus  groves  in  central  California  were  handi- 
capped from  the  start,  in  that  many  trees  were  originally  of  poor 
type.  The  buds  were  not  properly  selected,  the  rootstocks  were  of 
various  kinds  and  strains,  and  some  trees  were  essentially  runts  which 
had  remained  in  nurseries  until  reaching  salable  size. 

As  is  well  known,  sour  orange  is  more  resistant  to  disease  than 
other  root  stocks  commonly  used.  In  view  of  this  fact,  it  is  urgently 
advised  in  the  light  of  our  present  knowledge,  that  only  good  sour 
stock  be  used  in  planting  or  replanting  citrus  trees  on  adobe  or  other 
heavy  soils. 


BULL.  405]  CITRUS    CULTURE   IN    CENTRAL    CALIFORNIA  23 

It  is  true,  however,  that  sweet  stock  often  produces  larger  and 
more  vigorous  trees  than  sour  stock,  and  of  this  there  is  at  least  one 
excellent  example  in  Tulare  County. 

The  striking  variation  and  prevalence  of  inferior  strains  in  sour 
rootstocks,  as  shown  by  the  studies  of  Webber10  may  account  for  the 
poor  trees  in  certain  groves  of  this  stock.  At  present,  however,  we 
are  mainly  concerned  with  the  proper  management  of  old  groves 
already  established  on  stocks  susceptible  to  disease.  "Where  the  root 
injury  is  due  primarily  to  water  it  is  evident  that  the  first  step  is  to 
dry  out  the  ground.  Instances  have  been  noted  on  the  contrary  where 
the  ill-advised  practice  has  been  followed  of  making  short  furrows 
with  hoes  in  order  to  give  especially  heavy  irrigation  to  trees  already 
suffering  from  rotten  roots  caused  by  excessive  moisture.  Where 
the  diseased  trees  are  near  pipe  lines,  one  irrigation  furrow  in  the 
center  of  each  middle  may  be  used  to  get  the  water  past  the  trees 
which  are  on  water-logged  soil,  and  the  furrow  may  then  be  divided 
into  two  or  more  among  the  good  trees  farther  from  the  pipe  line. 
With  a  little  practice,  the  flow  of  water  can  be  kept  evenly  divided 
by  using  pipes  or  by  embedding  burlap  in  the  ground  at  the  junction 
of  the  furrows  to  prevent  washing  of  the  soil  and  consequent  shifts 
in  the  flow  of  water.  In  other  cases  the  use  of  one  furrow  in  the 
center  of  each  middle,  for  the  full  length,  may  be  advisable,  or  the 
tree  rows  may  be  irrigated  on  one  side  only,  and  then  on  the  other 
side,  after  the  drying  of  the  soil.  Summer  cover-crops  soon  dry  out 
the  soil  but  may  be  hard  on  unthrifty  trees  by  competing  for  plant 
food  materials.  Lack  of  aeration  appears  to  be  the  primary  cause  of 
root  rot,  consequently  the  cracking  of  adobe  soils  should  by  all  means 
be  encouraged  where  rotting  roots  are  present. 


10  Webber,    H.    J.      Selection    of    stocks    in    citrus    propagation.      California 
Agr.  Exp.   Sta.  Bull.   317:    1-32.      1920. 


15m-8,'26 


